The method of reverse osmosis (RO) is an effective and energy-saving method of desalination which is widely employed for obtaining water for industrial use, for agriculture, potable water and ultapure water. The method consists in applying mechanical (gauge) pressure over a saline solution, such as seawater, which is higher than the osmotic pressure of the same solution, in a volume delimited by a semi-permeable membrane (RO membrane). Thereby, the solvent (water) is squeezed through the membrane to its “permeate” side while dissolved salts remain in the solution at the “feed” side of the membrane.
Herein, osmotic pressures of solutions are referenced to pure solvent, i.e. if given saline water solution has osmotic pressure PO, this means, pure water from the other side of an osmotic membrane will permeate towards this solution as if under gauge pressure PO.
When the RO membrane is used for a long time for separation of salts, other components of the raw solution termed “foulants” herein, such as suspended particles, organic matter, colloids tend to accumulate on the “feed” surface of the membrane. Some dissolved salts also may precipitate on the surface, forming scale. A fouled membrane has reduced separability, increased pressure losses and therefore has to be cleaned.
It is known to clean the membrane by stopping the production process and pumping cleaning chemical solutions to wash the fouled surface. Such methods are costly, not very effective, and create new environmental problems.
A more effective method is using the direct osmosis by feeding saline solution to the fouled feed side of the RO membrane while supplying solvent (water) to the permeate side of the membrane. The higher osmotic pressure at the feed side sucks the water from the permeate side to the feed side of the RO membrane, whereby the water penetrates into the interface between the membrane and the accumulated foulant, and separates the foulant from the membrane surface.
Publications EP0768112 and JP2000-079328 disclose similar implementations of the direct osmosis method. It is suggested that when the membrane is cleaned, the production RO process is stopped, the saline solution is supplied to the feed side of the membrane essentially without pressure (or at least under pressure lower than the osmotic pressure of the solution), and the water is supplied to the permeate side of the membrane also without pressure. Thus, the cleaning water is sucked back to the feed side essentially under the net differential between the two osmotic pressures.
However, the typical RO membrane is a tight multi-layered structure with very narrow passages (see FIG. 5) where water flows very slowly. Some areas of the membrane are close to the channel supplying water, other areas are remote. On the other hand, the osmotic pressure of the typical saline solution—seawater or brine—is high, and the net differential is also high. The combination of these factors leads to very fast back-suction of water (“catastrophic” filtration) from the permeate side to the feed side through the close areas of the membrane. As a result, the water can not reach the remote areas of the membrane before it is entirely sucked in the close areas, while the concentration at both sides of the membrane in the close areas is soon equalized. The cleaning process, therefore, stops in a few seconds and the membrane cannot be cleaned uniformly.